N-acetyltransferase (NAT) is an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
that
catalyzes the transfer of
acetyl groups from
acetyl-CoA to
arylamines, arylhydroxylamines and arylhydrazines.
They have wide specificity for
aromatic amine
In organic chemistry, an aromatic amine is an organic compound consisting of an aromatic ring attached to an amine. It is a broad class of compounds that encompasses aniline
Aniline is an organic compound with the formula C6 H5 NH2. Consi ...
s, particularly
serotonin, and can also catalyze acetyl transfer between arylamines without CoA. N-acetyltransferases are cytosolic enzymes found in the liver and many tissues of most mammalian species, except the
dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
and
fox, which cannot acetylate
xenobiotic
A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compo ...
s.
Acetyl groups are important in the conjugation of metabolites from the liver, to allow excretion of the byproducts (
phase II metabolism
Drug metabolism is the metabolic breakdown of drugs by living organisms, usually through specialized enzymatic systems. More generally, xenobiotic metabolism (from the Greek xenos "stranger" and biotic "related to living beings") is the set o ...
). This is especially important in the metabolism and excretion of drug products (
drug metabolism
Drug metabolism is the metabolic breakdown of drugs by living organisms, usually through specialized enzymatic systems. More generally, xenobiotic metabolism (from the Greek xenos "stranger" and biotic "related to living beings") is the set o ...
). __TOC__
Enzyme Mechanism
NAT enzymes are differentiated by the presence of a conserved
catalytic triad
A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, li ...
that favors
aromatic amine
In organic chemistry, an aromatic amine is an organic compound consisting of an aromatic ring attached to an amine. It is a broad class of compounds that encompasses aniline
Aniline is an organic compound with the formula C6 H5 NH2. Consi ...
and
hydrazine substrates.
NATs catalyze the
acetylation of small molecules through a double displacement reaction called the ping pong bi bi reaction.
The
mechanism
Mechanism may refer to:
* Mechanism (engineering), rigid bodies connected by joints in order to accomplish a desired force and/or motion transmission
*Mechanism (biology), explaining how a feature is created
*Mechanism (philosophy), a theory that ...
consists of two sequential reactions.
In reaction one acetyl-CoA initially binds to the enzyme and acetylates Cys
68.
In reaction two, after
acetyl-CoA is released, the acetyl acceptor interacts with the acetylated enzyme to form product.
This second reaction is independent of the acetyl donor since it leaves the enzyme before the acetyl acceptor binds.
However, like with many ping pong bi bi reactions, its possible there is competition between the acetyl donor and acetyl acceptor for the unacetylated enzyme.
This leads to substrate-dependent inhibition at high concentrations.
Enzyme Structure
![NAT catalytic triad](https://upload.wikimedia.org/wikipedia/commons/d/d8/NAT_catalytic_triad.png)
The two NAT enzymes in humans are
NAT1 and
NAT2.
Mice and rats express three enzymes, NAT1, NAT2, and NAT3.
NAT1 and NAT2 have been found to be closely related in species examined thus far, since the two enzymes share 75-95% of their
amino acid sequence
Protein primary structure is the linear sequence of amino acids in a peptide or protein. By convention, the primary structure of a protein is reported starting from the amino-terminal (N) end to the carboxyl-terminal (C) end. Protein biosynthe ...
.
Both also have an
active site cysteine residue (Cys
68) in the N-terminal region.
Further, all functional NAT enzymes contain a triad of catalytically essential residues made up of this
cysteine,
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the d ...
, and
asparagine
Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
.
It has been hypothesized that the catalytic effects of the
breast cancer
Breast cancer is cancer that develops from breast tissue. Signs of breast cancer may include a lump in the breast, a change in breast shape, dimpling of the skin, milk rejection, fluid coming from the nipple, a newly inverted nipple, or a r ...
drug
Cisplatin
Cisplatin is a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, cervical cancer, breast cancer, bladder cancer, head and neck cancer, esophageal cancer, lung cancer, mesothelioma, ...
are related to Cys
68.
The inactivation of NAT1 by Cisplatin is caused by an irreversible formation of a Cisplatin adduct with the
active-site cysteine residue.
The C-terminus helps bind acetyl CoA and differs among NATs including prokaryotic homologues.
NAT1 and NAT2 have different but overlapping substrate specificities.
Human NAT1 preferentially acetylates
4-aminobenzoic acid
4-Aminobenzoic acid (also known as ''para''-aminobenzoic acid or PABA because the two functional groups are attached to the benzene ring across from one another in the ''para'' position) is an organic compound with the formula H2NC6H4CO2H. PABA i ...
(PABA),
4 amino salicylic acid,
sulfamethoxazole, and
sulfanilamide.
Human NAT2 preferentially acetylates
isoniazid
Isoniazid, also known as isonicotinic acid hydrazide (INH), is an antibiotic used for the treatment of tuberculosis. For active tuberculosis it is often used together with rifampicin, pyrazinamide, and either streptomycin or ethambutol. For la ...
(treatment for
tuberculosis
Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, i ...
),
hydralazine
Hydralazine, sold under the brand name Apresoline among others, is a medication used to treat high blood pressure and heart failure. This includes high blood pressure in pregnancy and very high blood pressure resulting in symptoms. It has been ...
,
procainamide
Procainamide (PCA) is a medication of the antiarrhythmic class used for the treatment of cardiac arrhythmias. It is classified by the Vaughan Williams classification system as class Ia; thus it is a sodium channel blocker of cardiomyocytes. In add ...
,
dapsone,
aminoglutethimide, and
sulfamethazine.
Biological Significance
NAT2 is involved in the
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
of
xenobiotic
A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compo ...
s, which can lead to both the inactivation of
drugs and formation of toxic
metabolites that can be
carcinogen
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
ic.
The
biotransformation Biotransformation is the biochemical modification of one chemical compound or a mixture of chemical compounds. Biotransformations can be conducted with whole cells, their lysates, or purified enzymes. Increasingly, biotransformations are effected w ...
of xenobiotics may occur in three phases.
In phase I, reactive and polar groups are introduced into the substrates. In phase II, conjugation of xenobiotics with charged species occurs, and in phase III additional modifications are made, with efflux mechanisms leading to excretion by transporters.
A
genome-wide association study
In genomics, a genome-wide association study (GWA study, or GWAS), also known as whole genome association study (WGA study, or WGAS), is an observational study of a genome-wide set of genetic variants in different individuals to see if any vari ...
(GWAS) identified human NAT2 as the top signal for
insulin resistance, a key marker of
diabetes
Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ...
and a major cardiovascular risk factor
and has been shown to be associated with whole-body insulin resistance in NAT1
knockout mice. NAT1 is thought to have an
endogenous role, likely linked to fundamental cellular metabolism.
This may be related to why NAT1 is more widely distributed among tissues than NAT2.
Importance in Humans
Each individual metabolizes xenobiotics at different rates, resulting from polymorphisms of the xenobiotic metabolism
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s.
Both NAT1 and NAT2 are encoded by two highly polymorphic genes located on
chromosome 8
Chromosome 8 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 8 spans about 145 million base pairs (the building material of DNA) and represents between 4.5 and 5.0% of the total DNA ...
.
NAT2
polymorphisms were one of the first variations to explain this inter-individual variability for
drug metabolism
Drug metabolism is the metabolic breakdown of drugs by living organisms, usually through specialized enzymatic systems. More generally, xenobiotic metabolism (from the Greek xenos "stranger" and biotic "related to living beings") is the set o ...
. These polymorphisms modify the stability and/ or catalytic activity of enzymes that alter acetylation rates for drugs and xenobiotics, a trait called acetylator
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...
. For NAT2, the acetylator phenotype is described as either slow, intermediate, or rapid. Beyond modifying enzymatic activity,
epidemiological studies
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evid ...
have found an association of NAT2 polymorphisms with various cancers, likely from varying environmental
carcinogen
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
s.
Indeed, NAT2 is highly polymorphic in several human populations.
Polymorphisms of NAT2 include the single amino acid substitutions R64Q, I114T, D122N, L137F, Q145P, R197Q, and G286E.
These are classified as slow acetylators, while the wild-type NAT2 is classified as a fast acetylator.
Slow acetylators tend to be associated with drug toxicity and cancer susceptibility.
For instance, the NAT2 slow acetylator genotype is associated with an increased risk of
bladder cancer
Bladder cancer is any of several types of cancer arising from the tissues of the urinary bladder. Symptoms include blood in the urine, pain with urination, and low back pain. It is caused when epithelial cells that line the bladder become ma ...
, especially among cigarette smokers.
Single nucleotide polymorphisms (SNPs) of NAT1 include R64W, V149I, R187Q, M205V, S214A, D251V, E26K, and I263V, and are related to
genetic predisposition to
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
,
birth defect
A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities ca ...
s, and other diseases. The effect of the slow acetylator SNPs in the
coding region predominantly act through creating an unstable protein that aggregates intracellularly prior to
ubiquitination
Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...
and degradation.
50% of the British population are deficient in hepatic N-acetyltransferase. This is known as a negative acetylator status. Drugs affected by this are:
* isoniazid
* procainamide
* hydralazine
* dapsone
* sulfasalazine
Adverse events from this deficiency include
peripheral neuropathy
Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or or ...
and
hepatoxicity. The slowest acetylator
haplotype, NAT2*5B (strongest association with
bladder cancer
Bladder cancer is any of several types of cancer arising from the tissues of the urinary bladder. Symptoms include blood in the urine, pain with urination, and low back pain. It is caused when epithelial cells that line the bladder become ma ...
), seems to have been selected for in the last 6,500 years in western and central Eurasian people, suggesting slow acetylation gave an evolutionary advantage to this population, despite the recent unfavorable epidemiological health outcomes data.
Examples
The following is a list of human
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s that encode N-acetyltransferase enzymes:
References
{{Portal bar, Biology, border=no
EC 2.3.1